1. Field of the Invention
The invention relates generally to lifting apparatus and more particularly to crane blocks.
2. Description of Related Art
Cranes have been used for mechanically lifting objects of all shapes, sizes and weight for centuries. Historically, cranes have included a boom and a pulley attached at the end of the boom. A rope or cable routed over the pulley is attached to an object to be lifted. In some cases, doubling pulleys are used in a lifting operation, as exemplified in U.S. Pat. No. 691,492 issued to Roney in 1902. By doubling the cable on two pulleys, the lifting burden can be eased to enable a crane to handle heavier objects more readily. Such pulley arrangements are also illustrated in U.S. Pat. No. 1,671,435 issued to McKissick in 1927 and U.S. Pat. No. 2,502,570 issued to Johnson in 1950.
Contemporary lifting cranes typically have a cable with a hook and a heavy ball combination attached thereto. The heavy ball serves as a weight at the end of the cable to help pull the cable out when there is no object attached to the cable. In telescoping boom type cranes, a weight at the end of the cable assists the cable when the boom is being retracted or extended. Cranes for lifting medium to heavy objects also have included crane blocks attached to the end of a crane hoisting cable. A hook is attached to the crane block which hook is coupled with straps, rings or ropes attached to the object to be moved. U.S. Pat. No. 3,917,334 issued to Koster illustrates an example of a type of crane block. Koster discloses a snatch block which has a pair of side plates, sheave plates therebetween, liner plates between the sheave plates, and spacers and pins to hold this plate combination together. A cable is attached to one of the pins while the other pin serves to support a large hoisting hook. Koster's snatch block can also be used with a sheave to double the lifting capacity of a crane. However, complete disassembly of his block and change of many parts is required to make this switch. Accordingly, disadvantageously Koster's crane block can only be used in a single mode, as it is easier to have two of his snatch blocks available at a lifting site rather than to interchange the sheave and lifting pin.
Other crane block arrangements are illustrated in U.S. Pat. No. 4,697,790 issued to Yang and U.S. Pat. No. 5,154,401 issued to Schramm et al. Yang discloses a pulley hoist including a hook with U-shaped anchor brackets and a sheave rotatably mounted between the legs on a shaft therebetween. A rope or cable from a power unit extends around the sheave and is hooked back onto the power unit.
Scramm et al. discloses a marine block for raising, lowering or adjusting sails which includes a fixed or swivel head, shackle and a ground and polished axle composed of 17-4 ph stainless steel sandwiched between titanium/titanium alloy side cheeks. An aluminum sheave with filament wound epoxy glass redial bushing with Teflon (.TM.) liner slide over the axle. Oven cured Teflon (.TM.) filled thrust washers bonded to the sides of the sheave turn against polished inner surfaces of the side cheeks. This arrangement provides a corrosion resistant block for use in competition sailing.
These cable block arrangements can only be used in a single mode capacity. A versatile crane block arrangement that can be used in a cable doubling arrangement or in a cable weighting arrangement would provide an advancement in the art.